This invention relates to a nickel-base thermocouple having improved EMF stability in the temperature range of 800.degree.F to 1000.degree.F.
Nickel-base alloys have been in use as thermocouple alloys for temperature measurement and control for some time. An accepted couple is the type K thermocouple so designated by ISA (Instrument Society of America). The positive thermoelement of the type K thermocouple is an alloy typically having the composition in weight percent of 9 Cr, 0.4Si, balance Ni (Ni/9Cr/.4Si). Typical trade names are Chromel (manufactured by Hoskins) and Tophel (manufactured by W. B. Driver Co.). The negative thermoelement of the type K thermocouple is a Ni base alloy typically containing 93 weight percent Ni with Al, Mn, Si, Fe, and Co additions. Typical trade names are Alumel (manufactured by Hoskins) and Nial (manufactured by W. B. Driver Co.).
Among the existing type K thermocouple alloys, the negative thermoelement is less stable (larger EMF deviations) and less oxidation resistant than the positive thermoelement when exposed to an elevated temperature such as 1800.degree.F. A negative thermoelement with improved oxidation resistance and EMF stability and a thermocouple incorporating this thermoelement are described in U.S. Pat. Nos. 3,820,983, and 3,776,781, issued to C. D. Starr and T. P. Wang on June 28, 1974 and Dec. 4, 1973, respectively.
The EMF of the positive type K thermoelement, a Ni/9Cr/.4Si alloy, has recently been found to change significantly when heated for a relatively short time (for example, one hour) in the temperature range of 800.degree.F to 1000.degree.F. This EMF change is not caused by the preferential oxidation of the alloy, but is believed to be associated with the short range ordering reaction of the nickel-chromium alloy which has been reported in the literature. Nordheim, R. & Grant, N. S., "Resistivity Anomalies in the Nickel-Chromium System," J. of the Institute of Metals, Vol. 82, p. 440 (1953-54); Taylor, A. and Hinton, K. G., "A Study of Order-Disorder and Precipitation Phenomena in Nickel-Chromium Alloy," ibid, Vol. 81, p. 451 (1952-53); Starr, C. D. and Wang, T. P., "The Electrical Characteristics of a Nickel-Chromium-Aluminum-Copper Resistance Wire," Proc. of Institution of Electrical Engineers, Vol. 104, p. 515, (1957). Herein, short range ordering means periodic arrangement of atoms in localized areas only of the crystalline lattice of the alloy. Normally, the thermocouple is heated rapidly to an elevated temperture such as 1800.degree.F and cooled rapidly to ambient temperature. Upon maintaining the thermocouple in the temperature range of 800.degree.F to 1000.degree.F, however, the chromium and the nickel atoms may arrange themselves in a periodic manner through diffusion in the crystalline lattice, leading to the undesirable changes in EMF and resistivity of the alloy.
Thus, the advantages of improved EMF stability of the negative thermoelement of U.S. Pat. No. 3,820,983 cannot be fully realized in thermocouples incorporating it, particularly in the temperature range of 800.degree.F to 1000.degree.F.
An improved positive thermoelement is described in Ser. No. 553,666, assigned to the present assignee. However, this composition does not exhibit EMF stability upon exposure to the 800.degree.F to 1000.degree.F range.